class Node:
    def __init__(self):
        self.next = None
        self.prev = None
        self.data = None


class FIFO:
    def __init__(self, max_length):
        self.max_length = max_length
        self.length = 0
        self.head = Node()
        self.tail = Node()

        self.head.next = self.tail
        self.tail.prev = self.head

    def is_empty(self):
        return self.length == 0

    def is_full(self):
        return self.length == self.max_length


    def unshift(self, data):
        assert not self.is_full()
        node = Node()
        node.data = data

        pNode = self.tail.prev
        pNode.next = node
        node.prev = pNode

        self.tail.prev = node
        node.next = self.tail
        self.length += 1

    def shift(self):
        assert not self.is_empty()
        node = self.head.next
        nNode = node.next

        self.head.next = nNode
        nNode.prev = self.head
        self.length -= 1

        return node.data

    def __iter__(self):
        node = self.head.next
        while node != self.tail:
            yield node.data
            node = node.next

    def __len__(self):
        return self.length

if __name__ == "__main__":
    l = FIFO(10)

    for i in range(0, 10):
        l.unshift(i)

    for i in l:
        print(i)
      
    try:
        print("The count: %d" % (len(l)))
        l.unshift(100)
    except Exception:
        print("The FIFO is full")
        data = l.shift()
        print("The first data is: %d" % (data))
        l.unshift(100)
        print("Access every element.")
        for i in l:
            print(i)
     
